If One Twin Has Cancer, Will the Other?

Even among identical twins who share virtually all their DNA, a cancer diagnosis in one twin does not mean the other will develop cancer too. The largest study on this question, tracking over 200,000 twin pairs across Nordic countries, found that when one identical twin was diagnosed with any cancer, the other twin’s lifetime risk of also developing cancer reached about 46%, compared with a baseline risk of roughly 32% in the general twin population. That means the co-twin’s risk is genuinely elevated, but more than half the time, the other twin never develops cancer at all. The answer is more nuanced than “yes” or “no,” and it shifts depending on the type of cancer, the twins’ shared environment, and a surprising amount of biological divergence that accumulates even between genetically “identical” people.

What the Largest Twin Studies Found

The most informative data on this question comes from the Nordic Twin Study of Cancer, which pooled records on more than 200,000 twin pairs from Denmark, Finland, Norway, and Sweden, with follow-up spanning decades. The researchers estimated that the overall heritability of cancer, meaning the share of variation in cancer risk attributable to inherited genetic factors, is about 33%.¹ That leaves roughly two-thirds of the variation down to environmental exposures, lifestyle, and biological randomness.

The practical numbers are worth sitting with. Among identical (monozygotic) twins, when one twin had cancer, the co-twin’s cumulative risk of any cancer by age 100 was 46%. For fraternal (dizygotic) twins, that number was 37%. In absolute terms, that translates to a risk that was about 14 percentage points higher for identical co-twins and about 5 percentage points higher for fraternal co-twins, compared with the overall cohort risk of 32%.¹ The gap between the identical and fraternal figures tells us genetics matters, but the fact that more than half of identical co-twins stayed cancer-free tells us genetics is far from destiny.

An earlier landmark study in the New England Journal of Medicine, using Scandinavian twin registries as well, had reached broadly similar conclusions: heritable factors played a statistically significant role in several common cancers, but for most cancer types, shared environment and non-heritable factors accounted for the majority of risk.²

The Cancer Type Makes a Big Difference

Not all cancers are created equal when it comes to genetic influence. The Nordic study found that heritability varied enormously from one cancer to the next. Some cancers have a strong genetic signal, meaning that if one identical twin develops it, the co-twin’s odds go up more substantially. Others are driven almost entirely by environment or chance, and a co-twin’s diagnosis barely moves the needle.

The cancers with the highest estimated heritability were:

• Skin melanoma: about 58% of the variation in risk was attributed to genetic factors.¹ An Australian twin study found that identical co-twins of someone with melanoma were nearly 10 times more likely to develop melanoma than would be expected by chance, while fraternal co-twins were only about twice as likely.³
• Prostate cancer: about 57–58% heritable, the highest among internal cancers. The genetic contribution stayed relatively constant across different ages, even as screening practices varied between countries.⁴
• Colorectal cancer: about 40% heritable. Identical co-twins of someone with colorectal cancer were roughly three times more likely to develop it than the general population, while fraternal co-twins were about twice as likely.⁵
• Breast cancer: around 27–31% heritable. Despite being one of the most common cancers, the genetic contribution is moderate, with the bulk of risk coming from non-genetic sources.¹

Cancers heavily driven by specific environmental exposures, such as lung cancer in smokers or cervical cancer related to viral infection, tend to show lower heritability. For those, what matters most is whether both twins shared the same exposure, not whether they shared the same genes.

Why Identical Twins Still Get Different Cancers

This is the part that surprises most people. If identical twins have the same DNA, why would one get cancer and the other not? The short answer is that identical twins are not quite as identical as the textbook version suggests, and even when their DNA is truly the same, their biology diverges over time in ways that matter for cancer.

They Are Not Perfectly Genetically Identical

A 2021 study showed that monozygotic twins differ by an average of 5.2 mutations that arise very early in embryonic development, before or shortly after the embryo splits into two individuals.⁶ About 15% of identical twin pairs carry a substantial number of these early mutations unique to one twin. On top of that, both twins continue accumulating different mutations in their body cells throughout life, driven by DNA replication errors, sun exposure, smoking, and other environmental insults.⁷ By middle age, each twin’s cells carry a unique constellation of acquired mutations that the other twin does not share.

Epigenetic Drift Pulls Them Apart

Beyond changes to the DNA sequence itself, the way genes are switched on and off changes over a lifetime. Researchers studying identical twins discordant for breast cancer (one twin had it, the other didn’t) found hundreds of positions in the genome where the chemical tags controlling gene activity differed between the affected and unaffected twin. In one study, the vast majority of these differences involved a loss of these chemical marks in the twin who developed cancer.⁸ A larger study examining 41 cancer-discordant identical twin pairs across multiple cancer types found subtle but detectable differences in blood-based chemical markers near tumor-suppressor genes.⁹

These epigenetic changes appear to reflect environmental exposures more than inherited genetics. A recent study of twin pairs discordant for breast cancer concluded that most of the chemical modifications associated with cancer risk were independent of genetic effects, suggesting they served as markers of environmental risk factors rather than inherited predisposition.¹⁰

The Immune System Diverges Too

A study of 210 healthy twins found that for the vast majority of immune parameters measured, over three-quarters, non-heritable influences accounted for more than half the variation. Some immune characteristics became more variable between twins with age, pointing to the cumulative effect of different infections, diets, and environments.¹¹ Since the immune system plays a critical role in detecting and destroying early cancer cells, differences in immune function between twins could easily translate into one twin’s body catching a nascent tumor that the other twin’s body misses.

Striking Cases of Discordance

Case reports bring the statistics to life. One well-documented family carried a mutation in the BRCA1 gene, which confers a high risk of breast and ovarian cancer. One identical twin in the pair had developed cancer twice over 27 years. Her twin sister, who carried the exact same BRCA1 mutation, remained completely healthy. Researchers examined whether differences in large-scale DNA rearrangements between the twins could explain the gap and found that they could not, leaving the cause of their dramatically different outcomes unexplained.¹²

A Czech study of five pairs of identical twins where one sister had breast cancer found that not a single pair was concordant for the disease. Even in the pair confirmed to carry a BRCA1 mutation, the twins developed different cancers: one got breast cancer at 54, the other ovarian cancer at 43.¹³ These are small studies, but they vividly illustrate that sharing a high-risk gene is not the same as sharing a diagnosis.

Childhood Leukemia Is a Special Case

There is one scenario where cancer in one twin is directly connected to cancer in the other, and it involves a biological mechanism that does not apply to adult cancers. In identical twins who share a single placenta (which most but not all identical twins do), pre-leukemic cells that arise in one twin can physically travel through shared blood vessels to the other twin before birth. This is not inherited susceptibility. It is a literal transfer of abnormal cells from one fetus to the other.¹⁴

Researchers confirmed this by showing that concordant twin pairs shared the same clone-specific leukemia mutations, mutations too unique to have arisen independently in both twins. The pre-leukemic cells seed the co-twin’s bone marrow, where they sit quietly until additional mutations accumulate after birth and convert them into full-blown leukemia. Importantly, the second twin does not always develop leukemia. The transferred cells sometimes remain dormant indefinitely, so even this direct cell-sharing route does not guarantee concordance.¹⁴ This mechanism is one of three recognized routes of cross-individual cancer transmission during pregnancy, alongside transmission of maternal cancer cells to the fetus and gestational choriocarcinoma.¹⁵

What Twin Data Means for Cancer Screening

If your twin has been diagnosed with cancer, the evidence is clear that your risk is elevated compared with the general population, and the elevation is meaningful. Researchers have argued that twin studies can be used to identify high-risk individuals who might benefit from earlier or more intensive screening.¹⁶ But elevated risk is not the same as inevitability. A 46% cumulative lifetime risk is higher than the general population’s 32%, yet it still means the odds lean toward not developing cancer.

For fraternal twins, the situation is closer to that of ordinary siblings. Family-based cancer risk data show that having a first-degree relative with cancer raises your risk at most cancer sites, with the strongest familial risks seen for cancers like Hodgkin’s disease, testicular cancer, thyroid cancer, and ovarian cancer.¹⁷ Fraternal twins share roughly 50% of their genetic variation, just like non-twin siblings, so their familial cancer risk is broadly comparable. The extra concordance seen in identical twins comes from that additional 50% of shared genetics, which nudges the risk higher but does not double it.

The practical takeaway for someone whose twin has been diagnosed: mention it to your doctor, discuss screening options, and recognize that your shared genetics is only one piece of the puzzle. The two-thirds of cancer risk that is not heritable means that your own exposures, habits, and luck play a larger role than any gene you share.

X-Chromosome Inactivation and Cancer Risk in Female Twins

One of the more unexpected mechanisms that can create cancer-risk differences between genetically identical female twins involves how the X chromosome is handled. Females have two copies of the X chromosome, but in each cell, one copy is largely silenced early in development. Which copy gets silenced in any given cell is partly random, and certain genes escape this silencing to varying degrees. Researchers studying twins found that genes known to be involved in tumor development, including DDX3X and KDM6A, showed variable escape from silencing between individual women and across different tissues.¹⁸ Because this variability exists even between identical twins, it could contribute to why one female twin develops a cancer and the other does not, adding yet another layer of biological divergence that goes beyond the DNA sequence both twins share.

This line of research is still in its early stages, but it underscores a broader theme: the further scientists look into what makes identical twins different from each other, the more sources of divergence they find. Mutations arising during embryonic splitting, accumulated damage from decades of different environmental exposures, drifting patterns of gene regulation, immune-system reshaping by different infections, and random variation in X-chromosome silencing all pile up. Each one is a small contribution on its own. Together, they are enough to make one twin develop cancer while the other never does.